The making of Cwm Idwal
The deposits and igneous rocks at Cwm Idwal were formed during the Ordovician period, around 450 million years ago. These rocks were formed by deposit movements at the bottom of the ocean as well as volcanic processes that formed layers of dust, ash and lava. These layers formed a stack on the sea bed as tectonic forces took effect. Around 40 million years ago, the gigantic tectonic shifts compressed and shaped these layers further as the tectonic plates collided and formed creasing waves in the rock.
The bottom of these waves are called synclines, and Idwal's syncline can be seen running up the 'Twll Du' at the end of Cwm Idwal. It would not be possible to see Cwm Idwal's syncline if it wasn't for the influence of the glaciers during the last Ice Age around 10,000 to 13,000 years ago.
Snow covered Cwm Idwal (© SNPA)
During the last Ice Age, a thick sheet of ice covered Snowdonia. These ice sheets were especially dense in valleys such as Cwm Idwal, Cwm Cneifion and Cwm Clyd and as they shifted down they scraped rocks from the back of the valleys and left great ravines behind which can be seen today.
As the glaciers melted, they left other deposits which formed into small hills such as the ones by the side of Llyn Idwal which are called 'Beddau Milwyr Ynys Prydain' (The legendary site of the graves of the great Brittonic soldiers). Another moraine formed alongside solid rock at the north side of the valley, this is what holds the water in Llyn Idwal. After the last Ice Age, the main influence on the landscape of Cwm Idwal has been climate and human influences.